Certain peroxidases (lactoperoxidase, myeoperoxidase) exert an antimicrobial effect when combined with H2O2 and an appropriate oxidizable substance such as thiocyanate, iodide, bromide or chloride ions. This antimicrobial system appears to be operative in saliva and neutrophilic polymorphonuclear leukocytes. This research is studying the properties, mechanism of action and biological significance of the peroxidase-mediated antibacterial systems with particular emphasis on the role of these systems in the neutrophil.